Slope stability is a critical safety and production issue for open cut mines, quarries, civil engineering works and the like. Major wall failures can occur seemingly without warning causing loss of lives, damage to equipment and significant disruption to the mining process resulting in significant losses in productivity.
Tell-tale signs of slope instability include the opening of cracks on the wall surface and crest, audible creaking, and increased rilling of spoil. It is difficult to interpret these signs to be able to distinguish between expected settling of recently excavated ground and events that will lead to catastrophic slope failure.
There are various slope monitoring systems employed by mine sites to monitor movement of slopes in order to provide an accurate reflection of the movement characteristics of the monitored slope. Such systems include the use of extensometers and laser electronic distance measurement to measure the dilation of cracks appearing on the crest or face of the slope. Geotechnical specialists can then interpret the pattern and history of movement to improve prediction of the failure process and to advise appropriate and timely stabilisation or safety management actions.
Almost all slopes exhibit a range of movement types prior to failure. These movement types include (T. D. Sullivan, “Understanding pit slope movements”, Geotechnical Instrumentation and Monitoring in Open Pit and Underground Mining p 435-445, 1993):                1) regressive movements leading to stability,        2) progressive movements leading to collapse,        3) transitional movements which combine the regressive movements followed by progressive, and        4) stick slip which is a number of regressive/transgressive movements normally induced by an external influence such as rainfall, blasting or mining.        
The Applicants have previously provided a novel slope monitoring system published under International Publication number WO 2002/046790. They have also described a Method and System of Determining Alarm Conditions that is particularly useful for their slope monitoring system in WIPO publication WO 2007/012112. The content of these two specifications are incorporated herein by reference.
More recently they have developed a Work Area Monitor described in International patent application number PCT/AU2011/001042. The Work Area Monitor device also uses slope stability radar to monitor the stability of a slope in a mine work area. Although existing alarm determination methods can be applicable to the Work Area Monitor the inventor has determined that other alarm methodologies can be suitable.
All absolute movement measures (displacement, velocity, acceleration and other time-derivatives) of a wall depend on many factors including the displacement type, the size of the moving area, the material type, the planes of weakness in the wall, complexity of the sliding plains, the temporal history of movements, and external influences on the system. Even the look angle of the monitoring apparatus influences the apparent current velocity of the movement. For example, if the look angle is 60 degrees from the wall movement velocity vector, the measured velocity will be half the actual velocity of the wall. In short, simply using an absolute movement measure to trigger alarms gives limited indication of the risk of failure associated with the slope under consideration.
Thus, whilst prior art slope monitoring apparatuses offer varying levels of monitoring accuracy, it is desirable to provide a slope monitoring apparatus that can automatically and accurately determine alarm conditions based on the recorded displacement data of the slope under inspection, thus providing a warning of a change in risk associated with the stability of a slope.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge in Australia.